package dnsforward

import (
	"bytes"
	"fmt"
	"io/ioutil"
	"log"
	"math/rand"
	"net"
	"net/http"
	"net/url"
	"strings"
	"sync"
	"time"

	"github.com/jedisct1/go-dnsstamps"

	"github.com/ameshkov/dnscrypt"
	"github.com/joomcode/errorx"
	"github.com/miekg/dns"
)

const defaultTimeout = time.Second * 10

type Upstream interface {
	Exchange(m *dns.Msg) (*dns.Msg, error)
	Address() string
}

//
// plain DNS
//
type plainDNS struct {
	boot      bootstrapper
	preferTCP bool
}

var defaultUDPClient = dns.Client{
	Timeout: defaultTimeout,
	UDPSize: dns.MaxMsgSize,
}

var defaultTCPClient = dns.Client{
	Net:     "tcp",
	UDPSize: dns.MaxMsgSize,
	Timeout: defaultTimeout,
}

// Address returns the original address that we've put in initially, not resolved one
func (p *plainDNS) Address() string { return p.boot.address }

func (p *plainDNS) Exchange(m *dns.Msg) (*dns.Msg, error) {
	addr, _, err := p.boot.get()
	if err != nil {
		return nil, err
	}
	if p.preferTCP {
		reply, _, err := defaultTCPClient.Exchange(m, addr)
		return reply, err
	}

	reply, _, err := defaultUDPClient.Exchange(m, addr)
	if err != nil && reply != nil && reply.Truncated {
		log.Printf("Truncated message was received, retrying over TCP, question: %s", m.Question[0].String())
		reply, _, err = defaultTCPClient.Exchange(m, addr)
	}

	return reply, err
}

//
// DNS-over-TLS
//
type dnsOverTLS struct {
	boot bootstrapper
	pool *TLSPool

	sync.RWMutex // protects pool
}

func (p *dnsOverTLS) Address() string { return p.boot.address }

func (p *dnsOverTLS) Exchange(m *dns.Msg) (*dns.Msg, error) {
	var pool *TLSPool
	p.RLock()
	pool = p.pool
	p.RUnlock()
	if pool == nil {
		p.Lock()
		// lazy initialize it
		p.pool = &TLSPool{boot: &p.boot}
		p.Unlock()
	}

	p.RLock()
	poolConn, err := p.pool.Get()
	p.RUnlock()
	if err != nil {
		return nil, errorx.Decorate(err, "Failed to get a connection from TLSPool to %s", p.Address())
	}
	c := dns.Conn{Conn: poolConn}
	err = c.WriteMsg(m)
	if err != nil {
		poolConn.Close()
		return nil, errorx.Decorate(err, "Failed to send a request to %s", p.Address())
	}

	reply, err := c.ReadMsg()
	if err != nil {
		poolConn.Close()
		return nil, errorx.Decorate(err, "Failed to read a request from %s", p.Address())
	}
	p.RLock()
	p.pool.Put(poolConn)
	p.RUnlock()
	return reply, nil
}

//
// DNS-over-https
//
type dnsOverHTTPS struct {
	boot bootstrapper
}

func (p *dnsOverHTTPS) Address() string { return p.boot.address }

func (p *dnsOverHTTPS) Exchange(m *dns.Msg) (*dns.Msg, error) {
	addr, tlsConfig, err := p.boot.get()
	if err != nil {
		return nil, errorx.Decorate(err, "Couldn't bootstrap %s", p.boot.address)
	}

	buf, err := m.Pack()
	if err != nil {
		return nil, errorx.Decorate(err, "Couldn't pack request msg")
	}
	bb := bytes.NewBuffer(buf)

	// set up a custom request with custom URL
	url, err := url.Parse(p.boot.address)
	if err != nil {
		return nil, errorx.Decorate(err, "Couldn't parse URL %s", p.boot.address)
	}
	req := http.Request{
		Method: "POST",
		URL:    url,
		Body:   ioutil.NopCloser(bb),
		Header: make(http.Header),
		Host:   url.Host,
	}
	url.Host = addr
	req.Header.Set("Content-Type", "application/dns-message")
	client := http.Client{
		Transport: &http.Transport{TLSClientConfig: tlsConfig},
	}
	resp, err := client.Do(&req)
	if resp != nil && resp.Body != nil {
		defer resp.Body.Close()
	}
	if err != nil {
		return nil, errorx.Decorate(err, "Couldn't do a POST request to '%s'", addr)
	}

	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, errorx.Decorate(err, "Couldn't read body contents for '%s'", addr)
	}
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("Got an unexpected HTTP status code %d from '%s'", resp.StatusCode, addr)
	}
	if len(body) == 0 {
		return nil, fmt.Errorf("Got an unexpected empty body from '%s'", addr)
	}
	response := dns.Msg{}
	err = response.Unpack(body)
	if err != nil {
		return nil, errorx.Decorate(err, "Couldn't unpack DNS response from '%s': body is %s", addr, string(body))
	}
	return &response, nil
}

//
// DNSCrypt
//
type dnsCrypt struct {
	boot       bootstrapper
	client     *dnscrypt.Client     // DNSCrypt client properties
	serverInfo *dnscrypt.ServerInfo // DNSCrypt server info

	sync.RWMutex // protects DNSCrypt client
}

func (p *dnsCrypt) Address() string { return p.boot.address }

func (p *dnsCrypt) Exchange(m *dns.Msg) (*dns.Msg, error) {

	var client *dnscrypt.Client
	var serverInfo *dnscrypt.ServerInfo

	p.RLock()
	client = p.client
	serverInfo = p.serverInfo
	p.RUnlock()

	now := uint32(time.Now().Unix())
	if client == nil || serverInfo == nil || (serverInfo != nil && serverInfo.ServerCert.NotAfter < now) {
		p.Lock()

		// Using "udp" for DNSCrypt upstreams by default
		client = &dnscrypt.Client{Proto: "udp", Timeout: defaultTimeout}
		si, _, err := client.Dial(p.boot.address)

		if err != nil {
			p.Unlock()
			return nil, errorx.Decorate(err, "Failed to fetch certificate info from %s", p.Address())
		}

		p.client = client
		p.serverInfo = si
		serverInfo = si
		p.Unlock()
	}

	reply, _, err := client.Exchange(m, serverInfo)

	if err, ok := err.(net.Error); ok && err.Timeout() {
		// If request times out, it is possible that the server configuration has been changed.
		// It is safe to assume that the key was rotated (for instance, as it is described here: https://dnscrypt.pl/2017/02/26/how-key-rotation-is-automated/).
		// We should re-fetch the server certificate info so that the new requests were not failing.
		p.Lock()
		p.client = nil
		p.serverInfo = nil
		p.Unlock()
	}

	return reply, err
}

func (s *Server) chooseUpstream() Upstream {
	upstreams := s.Upstreams
	if upstreams == nil {
		upstreams = defaultValues.Upstreams
	}
	if len(upstreams) == 0 {
		panic("SHOULD NOT HAPPEN: no default upstreams specified")
	}
	if len(upstreams) == 1 {
		return upstreams[0]
	}
	n := rand.Intn(len(upstreams))
	upstream := upstreams[n]
	return upstream
}

func AddressToUpstream(address string, bootstrap string) (Upstream, error) {
	if strings.Contains(address, "://") {
		url, err := url.Parse(address)
		if err != nil {
			return nil, errorx.Decorate(err, "Failed to parse %s", address)
		}
		switch url.Scheme {
		case "sdns":
			stamp, err := dnsstamps.NewServerStampFromString(address)
			if err != nil {
				return nil, errorx.Decorate(err, "Failed to parse %s", address)
			}

			switch stamp.Proto {
			case dnsstamps.StampProtoTypeDNSCrypt:
				return &dnsCrypt{boot: toBoot(url.String(), bootstrap)}, nil
			case dnsstamps.StampProtoTypeDoH:
				return AddressToUpstream(fmt.Sprintf("https://%s%s", stamp.ProviderName, stamp.Path), bootstrap)
			}

			return nil, fmt.Errorf("Unsupported protocol %v in %s", stamp.Proto, address)
		case "dns":
			if url.Port() == "" {
				url.Host += ":53"
			}
			return &plainDNS{boot: toBoot(url.Host, bootstrap)}, nil
		case "tcp":
			if url.Port() == "" {
				url.Host += ":53"
			}
			return &plainDNS{boot: toBoot(url.Host, bootstrap), preferTCP: true}, nil
		case "tls":
			if url.Port() == "" {
				url.Host += ":853"
			}
			return &dnsOverTLS{boot: toBoot(url.String(), bootstrap)}, nil
		case "https":
			if url.Port() == "" {
				url.Host += ":443"
			}
			return &dnsOverHTTPS{boot: toBoot(url.String(), bootstrap)}, nil
		default:
			// assume it's plain DNS
			if url.Port() == "" {
				url.Host += ":53"
			}
			return &plainDNS{boot: toBoot(url.String(), bootstrap)}, nil
		}
	}

	// we don't have scheme in the url, so it's just a plain DNS host:port
	_, _, err := net.SplitHostPort(address)
	if err != nil {
		// doesn't have port, default to 53
		address = net.JoinHostPort(address, "53")
	}
	return &plainDNS{boot: toBoot(address, bootstrap)}, nil
}